1. Field of the Invention
The present invention relates to thick carbon fiber precursor acrylic tows containing at least 20,000 filaments, with high quality and high productivity, as well as methods of producing and using the same.
2. Description of the Background
Demand for carbon fibers has increased in recent years, since they are widely used in premium applications, such as in airplane and sporting goods manufacture, and in general industrial applications typified by civil engineering. To satisfy the increasing demand of fibers for such applications, a drastic reduction in the cost of production of fibers, as well as an increase in fiber production capacity has been required. As a means for increasing the productivity of acrylic fiber tows as the precursor of carbon fibers, it has been found not to be effective to increase the total denier of the fibers by increasing the number of single fibers constituting the tows and to improve productivity per setup.
According to conventional methods of production, a spinning dope is guided into a coagulation bath to prepare coagulated tows. To guide and draw the tows, a plurality of rollers is used to transfer the tows before they are dried and compacted. However, when the total size of the tows is increased, the existing setups that are based on 12,000 filaments suffer from the disadvantage that the gap between the tows of adjacent weights becomes small and mutual interference and blending of the tows occur. As a result, damage of the single fiber, breakage, fluff and bonding, for example, occur and the process approval factor deteriorates. At the same time, a non-uniform size in a subsequent drawing processes invites non-uniformity of the size and also eventually, a deterioration of the properties of the resulting carbon fiber.
In order to prevent such a problem, the width of each roller must be widened in order to enlarge the gap between the tows of the adjacent weights. In this case, large modification of the setups, inclusive of a driving unit, must be made. If the roller is widened excessively, the guide operation of the tow and counter-measures to cope with problems become more difficult. These problems raise serious problems from the standpoint of safety.
Japanese Patent Laid-Open No. 5-195306 describes a method of controlling the tow width by using curved guides during the processing inside a bath. While this method allows for control of the tow width between the guides inside the bath, however, the problem of mutual interference and blending of the tows remains problematic on the rollers where problems are more likely to occur. Moreover, the weight variation ratio in the longitudinal direction of the tow made by prior art method is very large. As the result, the tensile strength in the longitudinal direction of the tow is not uniform. At the same time, the process approval factor deteriorates and non-uniform oiling occurs. The properties of the resulting carbon fiber deteriorate also.
Thus, a need continues to exist for a method of producing acrylic fiber tows for carbon fiber production which overcomes the above disadvantages.